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As #18412 points out, it should be OK for multiple case alternatives to have a higher rank type, provided they are all the same. This patch implements that change. It sweeps away GHC.Tc.Gen.Match.tauifyMultipleBranches, and friends, replacing it with an enhanced version of fillInferResult. The basic change to fillInferResult is to permit the case in which another case alternative has already filled in the result; and in that case simply unify. It's very simple actually. See the new Note [fillInferResult] in TcMType Other refactoring: - Move all the InferResult code to one place, in GHC.Tc.Utils.TcMType (previously some of it was in Unify) - Move tcInstType and friends from TcMType to Instantiate, where it more properly belongs. (TCMType was getting very long.)
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